1. Field of the Invention
The present invention relates to a motor control system and a motor control method which protect a tool and workpiece at the time of a power outage during operation of a machine tool.
2. Description of the Related Art
A numerical control machine tool controls a tool rotation motor which drives rotation of a tool (hereinafter referred to as a “tool motor”) and a workpiece motor which drives a workpiece by a numerical control device by numerical control (NC control) so as to machine a workpiece to a predetermined shape by the tool. Such numerical control machine tools include ones where the tool motor and workpiece motor have to be operated synchronously at all times. For example, a gear-forming machine which machines a workpiece to form a gear corresponds to this.
The motor control systems for synchronously controlling a tool motor and a workpiece motor include NC command synchronization systems and master-slave synchronization systems. An NC command synchronization system is a system in which two shafts are given movement commands for synchronous operation from the NC control device side and these commands are used for synchronous control. On the other hand, the master-slave synchronization system is a system in which a drive command is given from the NC control device side to one shaft (master shaft), position feedback information (pulses) of the master shaft is acquired, a drive command which is proportional to the position feedback information of the master shaft is given to the other shaft (slave shaft), and that command is used for synchronous control.
In the above-mentioned gear-forming machine, if a power outage occurs when the workpiece is being machined by a tool, the tool motor and the workpiece motor will decelerate and stop since the power to the amplifiers which drive them is cut. In this regard, the deceleration of the tool motor and the workpiece motor is deceleration which runs counter to the speed control from the numerical control device, so the operations of the tool motor and the workpiece motor until stopping become asynchronous. If, in this way, the synchronous operations of the tool motor and the workpiece motor become asynchronous operations due to a power outage, the workpiece and the tool are liable to be damaged.
Among general machine tools, there are ones wherein, at the time of a power outage, the drop in a direct current link (DC link) voltage of the alternating current/direct current converter of the machine tool is monitored while forcibly making the shaft of the machine tool decelerate, regenerated energy is used to maintain the DC link voltage, and a linear shaft for moving the tool or workpiece is made to retract. In this regard, in a gear-forming machine which employs the NC command synchronization system, if ignoring the NC command and forcibly making the shaft decelerate in accordance with the drop in the DC link voltage, synchronization with the shaft which operates coordinated with this will be lost, so this technique could not be applied at the time of a power outage.
Therefore, the applicant previously proposed a gear-forming machine which employs a master-slave synchronization system wherein when a power outage occurred during machining, the state of the drop in the DC link voltage was monitored while forcibly making the master shaft constituted by the tool shaft decelerate and making this and the slave shaft constituted by the workpiece shaft decelerate and stop while maintaining synchronization (see Japanese Patent Publication No. 8-227307A). Due to this technique, even if there is a power outage, the synchronization of the tool shaft and the workpiece shaft is substantially maintained. Note that, in the case of applying this art, the workpiece shaft may also be made the master shaft and the tool shaft may be made the slave shaft.
In this regard, even if using the motor control system which is disclosed in Japanese Patent Publication No. 8-227307A, if a power outage occurs right in the middle of cutting a gear and the master shaft is made to forcibly decelerate, the slave shaft cannot keep up with the forced deceleration of the master shaft, the synchronization error ends up becoming larger in an instant, and the workpiece is liable to end up being damaged. In particular, in the case of high precision gear forming, the damage to the workpiece has been an issue.